This invention relates to window structures that can be adjusted to fit window frames or recesses of different sizes by means of telescoping sides formed of angle merbers resiliently clamped together by edge clamps. In particular, the invention relates to an adjustable window structure that can easily be assembled and forced into place within the confines of a window frame or recess and can as easily be removed for storage and in which the edge clamps that hold the telescoping sides together also hold the edges of flexible sheet material constituting either a pane or screen.